are sensory receptors
within the belly of a muscle
, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system
via sensory neurons
. This information can be processed by the brain to determine the position
of body parts. Changes in length detected by muscle spindles also plays an important role in regulating the contraction of muscles, by preventing unwanted stretching. This property is demonstrated by the stretch reflex
Muscle spindles are found within the belly of muscles, embedded in extrafusal muscle fibers. They are composed of 3-10 intrafusal muscle fibers, of which there are three types:
Muscle spindles are encapsulated by connective tissue, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.
The muscle spindle has both sensory and motor components.
- Primary and secondary sensory fibers spiral around and terminate on the central portions of intrafusal fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons.
- In humans, the motor component is provided by gamma motoneurons; in many other species, beta motoneurons innervate the spindles. They cause a slight contraction of the end portions of the intrafusal muscle fibers when activated. The gamma (fusimotor) axons only innervate the intrafusal muscle fibers, whereas the beta (skeletofusimotor) axons innervate both extrafusal and intrafusal muscle fibers.
These motorneurons are classified as static or dynamic according to their pattern of innervation and their physiological effects.
- The static axons innervate the chain or bag2 fibers.
- The dynamic axons innervate the bag1 fibers and increase the velocity sensitivity of the Ia afferents.
The function of the gamma motor neuron neuromuscular junction is not to supplement the general muscle contraction provided by extrafusal fibers, but to modify the sensitivity of the muscle spindle to stretch. Upon release of acetylcholine by the gamma neuron, the end portions of the intrafusal muscle fibers contract, thus deliberately elongating the non-contractile central portions of intrafusal muscle fibers. This opens stretch-sensitive ion channels of the centrally-positioned sensory axons, leading to an influx of sodium ions. This raises the resting potential of these axons, thereby increasing the probability of action potential firing, thus increasing the sensitivity of the muscle spindle.
When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary sensory fibers (Group II afferent neurons) detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted monosynaptically to an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and polysynaptically through an interneuron to another alpha motoneuron, which inhibits contraction in the antagonist muscles.
PNF stretching, or proprioceptive neuromuscular facilitation, is a method of flexibility training that reduces the automatic reflex action in order to allow muscles to lengthen.
It is also believed that muscle spindles play a critical role in sensorimotor development